1. Field of the Invention
The present invention relates to a method for calibrating a color image forming apparatus such as a color printer or a color copier, and more particularly to a color signal conversion method for measuring a test chart outputted to improve the color stability of a color image forming apparatus using a chromaticity detecting means, and converting a color signal in a first color specification system that is the detection result of the chromaticity detecting means into a color signal in a second color specification system.
2. Background of the Related Art
Recently, there has been increasing demand for a color image forming apparatus of the electrophotography type or the inkjet type, such as a color printer or a color copier, having high color stability of its output image.
Thus, the color image forming apparatus having a sensor for detecting the chromaticity of a patch on the recording medium after the forming and fixing of a monochromatic gradation patch of cyan (C), magenta (M), yellow (Y) or black (K) or a mixed color patch in which CMY are mixed on the recording medium (hereafter referred to as a color sensor) is well known (e.g., refer to U.S. Patent Application Publication No. 2003/049040).
In this color image forming apparatus, the color stability of a final output image formed on the recording medium is controlled by feeding back the detected result to a calibration table for correcting the exposure amount, the process conditions and the color gradation characteristics of an image forming portion. Also, the output image of the color image forming apparatus may be detected by an external image reading apparatus or a chromaticity meter to make the same control.
This color sensor uses a light emitting element having three or more kinds of light sources with different emission spectra of red (R), green (G) and blue (B), respectively, and a light receiving element with sensitivity in the visible region, or a light emitting element having a light source that emits light of white color (W) and a light receiving element formed with three or more filters of different spectral transmittances. Thereby, three or more kinds of outputs such as the RGB outputs are obtained.
In the color image forming apparatus of the ink jet type, since the color balance changes depending on a change in the ink discharge amount with the lapse of time, an environmental difference from one place or time of use to another, or the individual differences among ink cartridges, the color gradation characteristics cannot be kept constant. Thus, some color image forming apparatuses effect color stabilization control by substituting a color sensor for the ink head and detecting the chromaticity of a patch on the recording medium.
In the above color stabilization control, there is a process of converting the sensor outputs, which are RGB values, into XYZ chromaticity values as defined by the International Commission on Illumination (CIE). For this conversion, the prior art has used a method employing a matrix, as well as a method using a look-up table.
However, the above prior art had the following problems.
Generally, the color matching functions for the spectral sensitivity of RGB outputs of the color sensor and the XYZ chromaticity values as defined by the International Commission on Illumination do not have completely linear relations. Therefore, there is a problem that some differences may occur between the XYZ chromaticity values obtained by converting the RGB outputs of the sensor and the XYZ chromaticity values obtained from the spectral reflectance as defined by the International Commission on Illumination.
That is, the XYZ chromaticity values calculated from the RGB output signals of the sensor for a certain patch and the XYZ chromacity values calculated from the spectral reflectance for the patch as defined by the International Commission on Illumination may be different in some cases. And this difference between the chromaticity values may vary in magnitude, depending on the color material or substratum color of the patch used in forming the patch.